1. Field of the Invention
The present invention relates to a linear amplifier, and particularly to a linear amplifier having good distortion and efficiency characteristics over semi-microwave and microwave bands which are employed in satellite communication systems, ground microwave communication systems and the like.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional linear amplifier disclosed in, for example, "6 GHZ Adaptive-Type Linearizer for Satellite Communication Earth Station HPA" by Sato and Kimura on pp. 10-195, at the national meeting of Institute of Electronics and Communication, 1986. Referring to FIG. 1, there are shown an input terminal 1, an output terminal 2, a high-powered amplifier (HPA) 5, a level control unit 17, a linearizer 18, a linearizer control unit 19 and a spectrum analyzer 20.
A description will now be made of the operation of the linear amplifier.
A signal inputted from the input terminal 1 is applied to the high-powered amplifier 5 via the level control unit 17 and the linearizer 18 and is outputted at output terminal 2. Portions of the input signal at terminal 1 and the output signal of high powered amplifier 5 are extracted by level control unit 17 and are applied to linearizer control unit 19.
The linearizer control unit 19 controls the linearizer 18 to make the levels of the input signal at terminal 1 and the output of HPA 5 equal to each other to perform its switching action, and supplies the input and output signals to the spectrum analyzer 20 which analyzes the frequency content of these signals.
The linearizer control unit 19 stores therein information about the frequency spectrum distribution of the components of the input/output signals measured by the spectrum analyzer 20 and performs arithmetic processing on the same, and thereafter supplies data for controlling the linearizer's characteristics to the linearizer 18 so that distortion components of the input/output signals are reduced.
The level control unit 17 includes a variable attenuator and a variable gain amplifier to correct the variation in level of each of the signals according to the control of the linearizer's characteristics so as to maintain a constant gain for the entire system comprising the level control unit 17, linearizer 18, and HPA 5.
The above conventional linear amplifier has a function for determining the difference between the frequency spectrum distribution of the input signal and that of the output signal of the high-powered amplifier 5 to automatically control the linearizer's characteristics such that the components of unnecessary distortion are reduced.
Since the conventional linear amplifier is constructed as described above, it is accompanied by the problem in that the spectrum analyzer, the linearizer control unit and the like are necessary components, so that the overall arrangement is constructed of large size, the operation efficiency is low because the high-powered amplifier is operated in a good linear region well below a saturation region and the operation rate is slow because data processing is effected by using digital circuitry applied to the control of the linearizer, thus leading to difficulty in the application of the linear amplifier to high-frequency band communication channels.